Electrical translating materials and method of making them



y .948. K. H. sToRKs ET AL ELECTRICAL TRANSLATING MATERIALS AND METHODOF MAKING THEM Filed July 28, 1945 2 Sheets-Sheet 1 FIG.

my a K. H. STORKS VEN TORS G. K EAL Q WL QE W OF MAKING THEM May 8,1948. K. H. STORKS ET AL ELECTRICAL TRANSLATING MATERIALS AND METHODFiled July 28, 1943 2 Sheets-Sheet 2 KH. STOKS INVENTORS GYMWM ATTORNEYPatented May 18, .1948

2,441,003 ELECTRICAL 'rnmsm'rma mums AND mrrnon or name rnnu 'Keith H.Storks, Basking mm, and Gordon K.

Teal. Summit,

laboratories. Incorporated,

N. 1., asailnors toBeli Telephone New York, N. Y., a v

corporation of New York Application July 28, 1943, Serial No. 496,414

Claims. (01. 175-368) This invention relates to electrical translatingmaterials and devices and to methods of making them.

The objects of the invention are to realize the optimum electricalcharacteristics for rectifying and other translating devices, includingan increased electrical efllciency; to obtain an improved and a moreuniform rectification material for use in these devices; to improvetheprocess by which this material is produced; and to effect otherimprovements in materials and devices of this character and in themethods by which they are manufactured.

With'the extension of signaling frequencies in the radio and allied artsinto the ultra-high frequency range where waves of a few centimeters inlength are employed for signaling purposes it has become necessary todevelop new types of apparatus for receiving, translating and utilizingthe signal energy at these extreme frequencies. Qne of the problems hasbeen to devise a satisfactory translating device which is capable ofdetecting, converting or otherwise translating signal waves havingfrequencies of the order mentioned. Up to the present time the mostpromising solution of this problem has been a translating or rectifyingdevice of the point-contact type. In one form a fine tungsten wire ismounted so that its free end engages the surface of an element havingsuitable rectifying properties, such as a crystal of elemental silicon.More specifically the silicon crystal elements of these prior rectifiershave been prepared by melting powdered silicon of relatively high purityin a furnace and cutting the resulting ingot into small wafers ofsuitable diameter and thickness. The crystal wafer is heated to apredetermined temperature in a reaction chamber, and a vaporous mixtureof allicon tetrachloride and hydrogen is introduced into the chamberunder I ditions. By carefully controlling the manufacturing parameters,that is, the concentration of the vapor mixture admitted to the chamber,the temperature .of the filament, the temperature of the chamber wallsand filament terminals during the deposition process and the durationand sequence of the deposition processes and heat treatments applicantshave found that marked improvements may be realized in the electricalperformance of the transmitting devices made from the material thusproduced.

Accordingly, a feature of the invention is a method of depositingsilicon on metal strips for use as translators in which a vapor mixtureincluding silicon tetrachloride of predetermined concentration isadministered to a reaction chamber containing a suspended metallicfilament, in which the filament is heated to a predetermined temperatureand maintained uniformly at this temperature during the decomposition ofthe silicon tetrachloride, and in which the walls of the reactionchamber and the terminals on which the filaments are suspended thenmounted on a, terminal block, and the fine tungsten wire is adjusted sothat its end makes a point contact with the surface of the crystal.

In accordance with the present invention it is possible to increase theefllciency and usefulness of these translating devices by means of atranslating element made by an improved process of depositing elementalsilicon on metallic filaments and of causing the filament material toalloy-with the silicon in a prescribed way. The translating element madeby this improved process consists of crystallized layers of a multiphasesystem of thermally deposited silicon and a suitable metal such astantalum, which may be supported on a backing strip of some suitablemetal, which may also be tantalum. In the process by which theserectifying elements are repared a strip or filament of tantalum is aremaintained at a relatively low temperature by a coolingsystem to preventthe formation of reaction products which otherwise tend to contaminatethe deposited silicon and to impair its usefulness. More specifically,the vapor administered to the reaction chamber is a mixture of silicontetrachloride and hydrogen, and the concentration of this mixture iscontrolled by a condenser associated with the cooling system of thereaction chamber in such a way that the partial pressure of the silicontetrachloride in the reaction chamber is always below the pressure ofsaturation. In this manner the silicon tetrachloride is prevented fromcondensing on the cool walls of the reaction chamber during thedeposition process.

Another feature of the deposition process is the method of cooling thereaction chamber in which the cooling liquid, after being raised to atemperature above that required for the condenser, is passed insuccession through a jacket surrounding the reaction chamber and'throughconduits forming the suspension terminals of the metallic filament onwhich the deposition istaking place.

Another feature of the invention is the method of preparing therectifier material in which suc-' cessive layers of silicon are closelyregulated con- 1 deposited on a metallie filament and in which eachlayer thus deposited is fused and crystallized before the succeedinglayer is deposited.

Another feature of the invention is the method of preparing rectifiermaterial in which the silicon is alloyed with the filament material in acontrolled manner to give a chemical system of a preferred composition,a preferred phase structure and a smooth surface contributing to lownoise output. This end is greatly aided by the controlled quenching ofthe alloy in flowing by.- drogen gas.

Another feature of the invention is a method whereby the thermal depositof the silicon is confined to one side of the metal filament or strip.This end is achieved by folding the strip so that only one side thereofis exposed within the reaction chamber.

Other features and advantages of the invention will be discussed morefully in the following detailed specification.

In the drawings accompanying the specification:

Fig. 1 illustrates a filament or strip (greatly enlarged) of backingmetal for the rectifying elements;

fore for obtaining silicon in suitable form for rec,-

tification elements having the Physical and electrical characteristicsessential to good performance in the ultra-high frequency range. One ofthe obstacles commonly encountered in these prior methods has been thedifficulty of attaining the high degree of silicon purity necessary forbest results. Experience shows that where metallurgical methods are usedto derive the silicon material it should have a purity close to 100percent. But the character and amounts of the small percentage ofimpurities are important and very difilcult to control. The strongaffinity which silicon in its elemental state has for other elementsmakes it extremely diflicult to obtain it in crystalline form in bodiesof substantial size and to exclude the presence of impurities in varyingand uncontrolled amounts.

Applicants method overcomes the difllculties above outlined by derivingthe elemental silicon from one of its compounds which can be readilypurified, depositing it in the form required and fusing the deposit tocrystallization under conditions which insure the desired degrees ofpurity and of alloying with a metal to give suitable electricalimpedance and rectification characteristics. More particularly, silicontetrachloride vapor mixed with hydrogen is administered to a reactionchamber where it is decomposed thermally to liberate the elementalsilicon which then deposits on a filament of metal, such as tantalum.However, experience shows that other reactions are likely to take placewithin the chamber when the temperature is raised to a value sufficientto cause decomposition of the silicon tetrachloride vapor. For example,reactions may occur between 4 the vapors within the chamber and thematerials of which the chamber walls and fixtures are. composed,resulting in products which deposit as impurities on the filament alongwith the liberated silicon. In applicants process these objectionablereactions are prevented by a novel method of controlling thetemperatureof the reaction chamber walls and fixtures in coniunctionwith the pressure of the vapor mixture administered to the chamber. Tothis end a vapor mixture of hydrogen and silicon tetrachloride having ahigh saturation of the tetrachloride is prepared in apparatus externalto the reaction chamber, and this mixture is passed through a refluxcondenser .on its way to the reaction chamber. A cooling liquid at apredetermined temperature passes through the condenser to reduce thesaturation of the vapor mixture to a definite value before the mixtureis permitted to enter the chamber. The cooling liquid after leavingthe'refiux condenser is raised to a higher temperature and is thencirculated through the walls and terminal fixtures of the reactionchamber to maintain them at the corresponding temperature during thedecomposition of the tetrachloride vapor. This increment between thetemperature I at which the saturation of the vapor mixture is fixedbefore its introduction into the chamber and the temperature of thechamber walls insures that the partial pressure of the tetrachloridevapor within the chamber is always less than the pressure of saturation.By this arrangement, therefore, it is possible to maintain thetemperature of the chamber walls and terminal fixtures sufficiently lowto prevent reactions taking place which would contaminate the silicondeposition and-yet sufliciently high to prevent condensation of thetetrachloride vapor within the chamber.

In the drawings Figs. 1 to 3 illustrate the metallic filament or ribbonwhich is used as the base for the thermal deposit of silicon. It will beunderstood, of course, that the invention is not limited to a particularmetal for the filament; the

particular metal or combination of metals found I to give theresultsdesired may be chosen. Ex-

v periments have been made with such metals such as tantalaum, platinum,tungsten and molybdenum and of these it is found that tantalum has theleast tendency to enter into solution with the silicon layer. Thismetal, therefore, gives excellent results where a high degree of siliconpurity is desired and where the alloying is subjected to control.

The filament I of the desired metal, such as.

tantalum, is formed with the requisite dimensions and is then preparedfor the reaction chamber shown in Fig. 8. This preparation consists infolding the ribbon along the middle, as indicated by the dotted line inFig, l, turning over and spotwelding the ends to form loops, andattaching thereto a pair of retractile springs 2 and 3 as illustrated inFig. 2. The purpose of folding the ribbon upon itself is to restrict thedeposite of the silicon to only one of the ribbon surfaces. After theribbon is thus prepared the head I of the reaction chamber 5 isunscrewed and removed, the circulating hose 8 being first. detached.molded or otherwise fixed in the head 4 are the terminal and coolingfixtures for suspending the filament I. These fixtures comprise a largeU- shaped tube 1 and a smaller U-shaped tube 8. These tubes arepreferably of copper, and they serve to conduct the cooling liquidthrough the heat distributing terminals 9 and I0, between which thefilament i is suspended by the retractile l is screwed back in place,

and 3. Following the suspension of the between the terminals 9 and I3the head closing the open end of the chamber 5. The cooling pipe 6, thepurpose of which will be explained hereinafter, is also attached asshown in the drawing.

The remaining apparatus shown in Fig. 8 is for the purpose ofadministering the vapor mixture to the reaction chamber and for heatingthe chamber to bring about the vapor decomposition. The hydrogen supplytank II which is connected by way of valve I2 and feed pipe I3 to a flowmeter I I. The purpose or the meter I4 is to maintain a uniform flow ofgas under varying external conditions. The hydrogen gas after passingthrough the regulating flow meter Il enters a trap I5 having liquidnitrogen therein for the purpose of trapping any mercury vapor that mayescape from the meter I4. Following the trap I5, the gas enters adeoxidizing furnace It for the purpose of removing any traces of freeoxygen that may be mixed with the hydrogen gas. On leaving the furnaceI6 the hydrogen and any water vapor that may be formed. in the furnaceenter the drying towers I1 and I8. These towers may be provided withphosphorous pentoxide for removing all traces of water vapor, thuspermitting only pure hydrogen to fiow into the outlet pipe I3.

The silicon tetrachloride vapor is derived from the vessel 23 where itis first mixed with the hydrogen gas under conditions that give themixture a high degree of saturation. The mixture is effected by leadingthe pure hydrogen gas through the inlet pipe 2I into the liquidtetrachloride. The hydrogen gas, after bubbling through the liquidtetrachloride, escapes through the outlet pipe 22 into the refluxcondenser 23. After leaving the condenser the vapor mixture passesthrough the intake pipe 24 and thence into the reaction chamber 5. Aby-pass pipe 25 is also provided with a valve 23 for diluting the vapormixture, if desired, by passing some of the hydrogen directly into thereaction chamber.

springs 2 filament a source 21 perature, preferably a few degrees belowroom temperature, by means of a heater 28. After passing through thethermometer chamberv 29, which gives a continuous indication of thetemperature, the liquid enters the reflux condenser 23 to fix theconcentration of the vapor emerging from the vessel 23. After leavingthe condenser 23 the liquid passes through a second heater 33 whichraises its temperature to a predetermined value, preferably about roomtemperature. From 33 the liquid passes'through thermometer chamber 3Iand thence into the cooling jacket 32 surrounding the walls of thereaction chamber 5. After passing through the cooling jacket 32 theliquid emerges into the pipe or hose 3. purpose of the pipe 6 is toreduce the temperature of the liquid which has been heated in the jacket32 by radiating or otherwise extracting some of the heat therefrom.After passing through the pipe 3 the liquid reenters the reactionchamber flows through the copper tube 1, throughthe terminal 9 andthence out of the chamber into a second cooling tube 33. From the tube33 the liquid reenters the chamber a second time and flows in the coppertube 3 through the terminal I3 and finally out by way of the waste pipe34. g

The necessary heat for producing the vapor decomposition within thechamber is derived from a source of current 35, and a suitable regulatorThe 6 33 serves to maintain the voltage necessary to produce therequired temperature within the chamber. The energy from the source 35is supplied to the suspended filament I within the chamber by connectingthe secondary winding of the transformer 31 to the circulating pipes Iand 8, thus causing current to flow through the filament by way ofterminals 3 and I3 and suspension springs 2 and 3.

The sequential steps of the process will now be described in detail,assuming specific temperatures values and other factors which have beenfound to give good results. A strip I of annealed tantalum is cut tosuitable length and folded as g 3. Before folding, the strip may bewashed if necessary in acetone to remove any grease that may be present.Also a matte finish may be given the surface of the strip by blasting itwith fine mesh silicon carbide. After thorough cleaning, the strip isfolded as above explained, and springs 2 and 3 are attached either bywelding or as indicated in the drawing. The prepared filament I is thenmounted within the reaction chamber as shown in Fig. 8.

Dry tank hydrogen is now passed over the filament for several minutes.To this end the valves I2 and 23 are opened and valves 33 and 33 areclosed. Hydrogen from the tank II passes through the flow meter I,through the liquid trap I5, furnace I6 and thence through the dryingtowers I1 and I3 into the supply pipe I3. From thence it flows throughthe by-pass pipe 25 into the reaction chamber and out through the wastepipe 43. While hydrogen is flowing through the chamber, the switch 4| isclosed and the filament I is raised to a temperature of 1203 C., whereit is maintained for a period of about thirty seconds. Thereupon thefilament is given a flash to the melting point of silicon, followingwhich it is permitted to cool rapidly to room temperature. Thetemperature of the filament within the chamber is determined by anywellknown means, such as an optical pyrometer.

The filament is now ready for the deposition process which consists indepositing a plurality, preferably three, separate layers of silicon,each layer being fused before the succeeding layer is deposited.Preparatory to the first step in the deposition process the valve 39 isopened and the openings of valves 23 and 33 are adjusted to give thedesired ratio between the amount of hydrogen flowing through the by-passpipe 25 and the amount flowing through the pipe 2i into thetetrachloride vessel 23. This ratio, which may be of the order of 1:1,may be determined by any suitable method, one of which is to observe therate at which bubbles emerge from the liquid tetrachloride in the vessel23. The flow of hydrogen in the meter I4 is fixed at 35 millimeterspressure of mercury across the capillary. The heater 28 is adjusted tomaintain the water entering the condenser 23 at a temperature about 3 or4 degrees below room temperature, which may be assumed to be 67 F., andthe heater 33 is adjusted to maintain the liquid emerging from thecondenser at 6'7 F.

After several minutes of flow to establish the equilibrium of the vapormixture of hydrogen and silicon tetrachloride the temperature of thefilament is raised to 935 C., and is maintained at that value for aboutten minutes. During this period the silicon tetrachloride vapor isdecomposed thermally, the liberated silicon is deposited on the surfaceof the filament I, and the unseen in Figs. 2 and which it is quenched inthe hydrogen,

wanted products of the reaction are discharged through the waste pipe40.

If it is assumed that the hydrogen gas emerging from the vessel 20 ishighly saturated, possibly super-saturated, with silicon tetrachloride,the condensation occurring within the condenser 23 will reduce theconcentration of tetrachloride to the saturation value corresponding tothe temperature within the condenser. When, therei'ore,'the mixtureenters the reaction chamber, it encounters a chamber wall temperaturewhich is somewhat higher than that of the condenser. This differentialincreases the saturation pressure of the vapor to a point which issafely above the partial pressure of the tetrachloride and therebyprevents condensation of the tetrachloride on the walls of the chamber.The same differential temperature relation is maintained between thecondenser and the terminal fixtures within the chamber by means of thecooling pipes I and 8 and the external cooling tubes 6 and 33 throughwhich the liquid is circulated before it finally emerges from the wastepipe 34.

After deposition has proceeded for a period of about ten minutes asdescribed, the fiow of tetrachloride vapor is stopped by closing thevalves 38 and 39, permitting the continued flow of hydrogen into thedeposition chamber. After about three minutes the filament temperatureis raised to 1200 0., and maintained there for two minutes. Thereuponthe temperature is quickly raised to the melting point of silicon,following which is a good heat conductor, to about 895 (1., forcing thefused silicon to rapidly crystallize.

The second and third layers of silicon are deposited, fused and quenchedunder conditions the same as those above described for the deposition ofthe first layer.

By this method pure silicon is deposited on the tantalum, a diffusion ofthe tantalum into the silicon takes place to form a polyphase structure,and the degree of the alloying action is closely controlled by quenchingthe fused structure in hydrogen gas. In this way a superior rectifyingmaterial is obtained.

The filament is now removed from the chamber and is cut into rectanglesor discs as illustrated in Figs. 5 and 6 for use in the rectifierassemblies.

assembled rectifier, greatly enlarged, is illustrated in Fig. '7. One ofthe silicon units 42 is included in the assembled device and is mountedon the threaded metallic stud 43. The mounting of the silicon unit onthe stud 43 may be accomplished by any suitable method. For example, thetantalum side of the unit 42 may be welded to the surface of the stud43. Following the attachment of the silicon unit 42 to the stud 43,which is integral with the metallic base member 44, the stud is screwedinto one end of the ceramic insulating cylinder 45. In like manner thethreaded stud 46, which is integral with the metallic cap 41, screwsinto the opposite end of the cylinder 45. The cap member 41 contains acentral bore for receiving the cylindrical metallic contact holder 48;and this holder is adjusted within the bore until the tip end of thetungsten contact wire 49, the opposite end of which is soldered in theholder 48, makes contact with the silicon surface of the element 43.When the desired degree of force is applied to the contact engagement ofthe wire 49 with the silicon element 43, the set screws 50 are tightenedto seize the holder 48.

The vessels and interconnec ing pipes forming 8 are of glass. The pipeare preferably of bronze, and the valve 8 the system shown in Fig.unions between sections of copper, the valves are of packing isprotected from the vapors in the system by lead washers on the valvestem. These precautions are taken to eliminate any source ofcontamination and to prevent any unwanted substance from finding its wayinto the reaction chamber.

The tank 5| contains nitrogen which may be used for flushing the systemto clean out impurities and to prevent the entry of oxygen, water vapor,or other substances.

What is claimed is:

1. The method of making an electrical translating element whichcomprises suspending a metallic filament in a reaction chamber,administering a vapor mixture including silicon tetrachloride to saidchamber, heating said filament to a, predetermined temperature foreffecting the decomposition of the silicon tetrachloride and thedeposition of a layer of silicon on said filament, cooling said reactionchamber to a predetermined temperature to prevent the contamination ofsaid silicon layer by the formation of undesired reaction products, andcontrolling the pressure of the vapor mixture before it enters thereaction chamber to prevent the condensation of the silicontetrachloride within said chamber.

2. The method of making an electrical translating element whichcomprises suspending a metallic filament in a reaction chamber,administering a vapor mixture including silicon tetrachloride to saidchamber, heating said filament to a predetermined temperature foreffecting the decomposition of the silicon tetrachloride and thedeposition of a layer of silicon on said filament, cooling said reactionchamber to a predetermined temperature to prevent the contamination ofsaid silicon layer by the formation of undesired reaction products, andestablishing a differential between the saturation pressure of the vapormixture before d after it enters the reaction chamber to prev% thecondensation of the silicon tetrachloride thin said chamber.

3. The method of making an electrical translating element whichcomprises suspending a mecon tetrachloride and the walls of said chamberand the consequent formation of contaminatin products, and controllingthe concentration of the silicon tetrachloride in said vapor mixturebefore it enters the reaction chamber to prevent the condensation of thesilicon tetrachloride within said chamber.

5. The method of making an electrical translating element whichcomprises suspending a metallic filament between two terminals in areaction chamber, passing a vapor mixture including a silicon compoundand a gas through a condenser and into said chamber, applying a voltageto said terminals for heating said filament to a predeterminedtemperature for effecting the decomposition of the silicon compound andthe deposition of a layer of silicon on said filament, passing a streamof cooling liquid first to said condenser and thence in series throughthe cooling jacket of said reaction chamber and through said suspendingterminals, controlling the temperature of said liquid at a given valueas it passes to said condenser to fix the concentration of said vapormixture, and controlling the temperature of said liquid at a. highervalue as it passes to said cooling jacket and terminals for increasingthe saturation pressure of said vapor mixture and preventing thecondensation of the silicon compound on the walls of said reactionchamber and on said suspending terminals.

6. The method of making an electrical translating element whichcomprises suspending a metallic filament in a reaction chamber,administering a vapor mixture including a compound of silicon to saidreaction chamber, heating said filament to a predetermined temperature anumber of times in succession for effecting the decomposition of thesilicon compound and the deposition of a corresponding number ofseparate layers of silicon on said filament, heating said filament to ahigher temperature, following the deposition of each layer, to fuse,crystallize, and to quench the deposited layer to give a desired phasestrucstrip at its middle to conceal one of its surfaces and to leave theother surface exposed, suspending said folded strip in a reactionchamber, administering to said reaction chamber a vapor mixtureincluding a compound of silicon, heating said metallic strip to apredetermined temperature to effect the thermal decomposition of saidsilicon compound and the deposition of a layer of silicon on the exposedsurface of said metallic strip, cutting said strip with the depositedsilicon thereon into units, and utilizing the metallic surface of saidunits for mounting them.

8. The method of making rectifier material which comprises fusingsilicon in the presence of tantalum to cause a diffusion of thetantaluminto the silicon and the formation of a polyphase structure, andquenching said structure to control the degree of diffusion.

9. The method of making rectifier material which comprises fusingsilicon in the presence of a body of tantalum to form a polyphasestructure of silicon and difiused tantalum on said tantalum body, andquenching the fused structure in hydrogen.

,10. The method of making rectifier material which comprises depositinga plurality of successive layers of silicon on a body of tantalum,fusing each successive layer to effect a diffusion of the tantalum intothe deposited silicon, and quenching each fused layer in succession tocrystallize the silicon and to control the degree of diffusion.

KEITH H. STORKS. GORDON K. TEAL.

REFERENCES CITED UNITED STATES PATENTS ture and smooth surface, andcooling said reaction chamber to a predetermined temperature to preventthe contamination of said silicon layers by the formation of undesiredreaction products.

7. The method of making an electrical translating element whichcomprises folding a metallic Date June 22, 1909 Feb. 5, 1935 Name VonBolton Feussner et al FOREIGN PATENTS Number Number

